Unlocking new possibilities

While faulty AI is easily recognised, an AI system that is doing its job often goes unnoticed. No one stops halfway through a level to admire the idiosyncrasies displayed by NPCs unless they are doing something completely out of character--the more unremarkable, the better the AI system. While achieving this result is still a priority for game developers, making games with an AI system that stands out for being good is a relatively new concept: few studios want to dedicate costly man-hours to chasing innovation in a highly technical field that, for the most part, is likely to go unnoticed. However, there are some exceptions. In 2007, AiGameDev.com launched its annual game AI awards, nominated and voted by the site's community. The purpose of the awards was to spotlight the games that showed promise in the field of AI, either by trying something different or exhibiting technical proficiency. In 2009, the Best Combat AI and the overall Best Game AI awards were won by the same studio--Guerrilla Games for Killzone 2. Remco Straatman, lead AI programmer at Guerrilla, says a lot has changed in game AI in the last five to 10 years, with more developers trading low-level scripting for more advanced NPC decision systems.
"In general, I think game AI has gone from the stage where it was an achievement if it did not stand out negatively to the point where AI in most big games is solid, and some titles are using innovative new ideas," Straatman says. "More development teams have also moved from simple state machines to behaviour trees and using planners in NPC AI systems describing knowledge of the world around the NPCs have improved with better knowledge for navigation over changing terrain, and more knowledge about strategic properties of the world such as cover. I also think advances in animation systems with better ways to combine various animations and physics have become available, which now allows for more realistic movement and responses to being hit [in combat AI]. Most of these systems were not around 10 years ago or simply could not run on the hardware available."
Creating a solid game AI system involves successfully networking smaller systems together. For example, a system that deals with the problem-solving capabilities of individual NPCs goes hand in hand with a system that makes sense of the gameworld and its parameters and helps NPCs make relevant decisions. Thankfully, developers don't have to build these systems from scratch: they use specific planners that generate increasingly complex networks.
"At the moment [Guerrilla Games] is using a specific type of planner for our NPCs called Hierarchical Task Network (HTN)," Straatman says. "This is capable of generating more complex plans than what we had before Killzone 2. We also keep on improving things like the CPU performance, which means we can support more NPCs in Killzone 3 than we could in Killzone 2. The terrain-reasoning systems we generate have also evolved over our various titles. We are now able to deal with much more dynamic terrain (like obstacles moving around or changing shape) than ever before. Our data on where there is cover has also become more detailed, something that allows NPCs to deal with more complex environments such as multistory buildings,